TENSIONS between Russia and Ukraine ratcheted up even further today, as Ukrainian leaders claimed their neighbours have warned of an imminent attack.

We shall have to bring to bear diplomatic, political, economic and other pressures in order to make this point

David Cameron

Ukraine's military reported the delivery of an ultimatum from Russia, urging them to surrender in Crimea by 3am (GMT) or face a military assault.

The crew of two Ukrainian warships in Crimea were told to surrender or be stormed, a spokesman said.

However, a Russia defence ministry spokesman in Moscow has since dismissed the reports as "utter nonsense", but refused to elaborate.

Russian troops now control all Ukrainian border posts in Crimea, after armed men first entered the region on Friday.

Reports suggest there are 6,000 Russian troops in Crimea.

Ukraine now fears Russia could move beyond the strategic peninsula and target and seize other pro-Russian parts of the country.

Military personnel, believed to be Russians, in Crimea [REUTERS]

Prime Minister David Cameron today warned Russian President Vladimir Putin his country will face diplomatic, political and economic pressure in response to the violation of Ukraine's sovereignty.

Mr Cameron earlier chaired a meeting of the National Security Council to discuss the growing crisis.

He said the world needed to send a "clear message" to Moscow.

He added: "What we want to see is a de-escalation rather than a continuation down the path that the Russian government has taken, violating the sovereignty and territorial integrity of another country.

"So we shall have to bring to bear diplomatic, political, economic and other pressures in order to make this point.

"That is the very clear message the whole world needs to send to the Russian government."

Mr Hague said Moscow should be in no doubt that it faced "significant costs" for "taking control" of the Ukrainian region.

Speaking on BBC Radio 4's Today programme, Mr Hague praised the Ukraine government for showing restraint despite the "provocation", and said he was "very concerned" about action by Russia in other parts of the country.

"If Russia continues on this course we have to be clear this is not an acceptable way to conduct international relations," Mr Hague said.

"That is something that Russia has to recognise...There will certainly be significant costs.

"There are things that we can do about it and must do about it."

Mr Hague said the diplomatic response was already happening, and signalled that economic sanctions were on the table.

"The world cannot just allow this to happen," he said.

He added: "The urgent thing is to get Russia and Ukraine to be in direct communication with each other."

Mr Hague said there was "no justification" for the action in Crimea, where he said Russia had "in effect taken control".

"Clearly we are very concerned about any possibility of a further move by Russia in other parts of Ukraine," he said.

"There are Ukrainian forces as well as Russian forces in Ukraine. There is a constant risk of miscalculation, of a flashpoint arising there (the Crimea) or in other parts of Ukraine."

He called on Russia to withdraw its troops to their barracks in Ukraine.

"Russia is entitled under its arrangements with Ukraine and the Black Sea fleet basing arrangements to have troops and naval forces in Crimea," he said.

"But when they are outside their bases, they are meant to operate with the agreement of the Ukrainian authorities.

The Foreign Secretary described the situation as "certainly the biggest crisis in Europe in the 21st century".

He dismissed suggestions that the Iraq war undermined the moral authority of the West to criticise Russia.

Earlier, Mr Hague walked to Independence Square in Kiev, site of mass protests against ousted president Viktor Yanukovych, before speaking to people manning the barricades and laying flowers, joining growing floral tributes to those who have died in the protests.

He is due to meet the acting president and prime minister of the country later today.

The G7 has suspended preparations for June's G8 summit in Sochi following the escalation of military action and condemned Russia's "clear violation of the sovereignty and territorial integrity of Ukraine".

Mr Cameron "remains gravely concerned" about the situation and, in a conversation with US president Barack Obama last night, agreed on the "urgent need for de-escalation" and for Russia to engage in a direct dialogue with Ukraine.

Pro-Russian troops took over a ferry terminal on the easternmost tip of Crimea close to Russia today, exacerbating fears that Moscow is planning to bring even more troops into the region.

Yesterday evening, the White House issued a joint statement on behalf of the G7 saying they are suspending participation in the planning for the upcoming summit because Russia's advances in the Ukraine violate the "principles and values" on which the G-7 and G-8 operate.

In Kiev, Moscow and other cities, thousands of protesters took to the streets to either decry the Russian occupation or celebrate Crimea's return to its former ruler.

"Support us, America!" a group of protesters chanted outside the U.S. Embassy in Kiev and one young girl held up a placard reading: "No Russian aggression!"

"Russia! Russia!" the crowd chanted in Moscow.

So far, however, Ukraine's new government and the West have been powerless to counter Russia's tactics.